Missons CTD

HiHaving installed FSX now and pretty much installed all the tweaks discussed in various parts of the forums, I am enjoying my FSX experience - however I am getting a pretty much repeatable CTD.If I load up FSX, select a mission and fly it, and then return to the missions screen, select another mission instead of the loading progress bar I get a black screen - I can then alt-tab to bring up the loading progress bar - which will then go through to 99% and then CTD (or very occasionally I get a black screen with all the mission sound but no picture which needs a task kill to shut it down).If I select free - flight and load up a flight, then no problems.I have got the latest version of all drives, direct X etc.Anyone else come across this?My System - Core2Duo E6600, 2gb Memory, Nvidia 7900GS 256mb GraphicsThanksJames